Compact isochronal cyclotron

ABSTRACT

A superconducting or non-superconducting compact isochronic cyclotron in which the particle beam is sectorally focused. The cyclotron comprising a solenoid forming a magnetic circuit which is energized by at least one pair of circular main coils surrounding the solenoid poles. The magnetic circuit includes at least three pairs of sectors known as &#34;hills&#34;, where the air gap is reduced, separated by sector-shaped spaces known as &#34;valleys&#34;, where the air gap is larger. The air gap between the hills has a substantially elliptical curved profile which completely closes off the radial end of the hills at the center plane.

SUBJECT OF THE INVENTION

The present invention relates to a cyclotron of novel design in whichthe particle beam is focused by sectors. More particularly, the presentinvention relates to an isochronal cyclotron comprising an electromagnetconstituting the magnetic circuit which includes at least three pairs ofsectors called "hills" where the air gap is smaller, these beingseparated by spaces in the form of sectors called "valleys" where theair gap has a greater dimension.

The present invention relates more particularly to a compact isochronalcyclotron, that is to say one energized by at least one pair of maincircular coils surrounding the poles of the electromagnet.

The present invention relates both to superconducting andnon-superconducting cyclotrons.

STATE OF THE ART

Cyclotrons are particle accelerators used in particular for theproduction of radioactive isotopes.

The cyclotrons are normally composed of three separate main assembliesconstituted by the electromagnet, the high-frequency resonator and thevacuum chamber with pumps.

The electromagnet guides the ions over a trajectory representingapproximately a spiral of radius which increases during theacceleration.

In modern cyclotrons of the isochronal type, the poles of theelectromagnet are divided into sectors having, alternately, a smallerair gap and a larger air gap. The azimuthal variation in the magneticfield which results therefrom has the effect of focusing the beamvertically and horizontally during the acceleration.

Among isochronal cyclotrons, it is convenient to distinguish cyclotronsof the compact type which are energized by at least one pair of maincircular coils and cyclotrons called separate-sector cyclotrons wherethe magnetic structure is divided into entirely self-contained separateunits.

First-generation isochronal cyclotrons are cyclotrons which use circularcoils of conventional type, that is to say non-superconducting coils.For these first-generation cyclotrons, the mean induction field obtainedwas limited to values of 1.4 tesla.

One particularly favourable embodiment for a cyclotron of this type isdescribed in Patent Application WO-A-86/06924 where the air gap of thesectors called hills is reduced to a value close to the size of theaccelerated beam, whereas the air gap of the sectors called valleys,which separate the hills, is very large so that the magnetic field y isapproximately zero.

Another particularly favourable embodiment of an isochronal cyclotronfocused by sectors is described in the document WO-A-91/07864 where thehills are coincident with the accelerator system by choosing theirconfiguration and dimensions appropriately.

Both documents have constant air gaps between hills.

The document U.S. Pat. No. 2,872,574 describes an isochronal cyclotronwhose air gap between hills has a profile which decreases linearly. Thiscyclotron is intended for accelerating particles up to a few tens of MeVproton.

The document IEEE Transaction on Nuclear Science (Vol. NS-32, No. 5/2,October 1985, NY-US, pp. 3316-3317) describes a compact isochronalcyclotron enabling H⁻ particles to be accelerated up to an energy of 30MeV for magnetic inductions between the hills of the order of 1.7 tesla,and in which the air gap between hills has a profile which increases upto a maximum value and decreases beyond that.

Over the last twenty years, cyclotrons called second-generationcyclotrons have appeared which use superconductor technologies. In thesecyclotrons, the main coils are of the superconducting type and enablemean inductions lying between 1.7 and 5 tesla to be obtained, whichmakes it possible to deliver particle beams having magnetic strengths(Br) markedly greater than those delivered by first-generationcyclotrons.

However, because of the higher inductions obtained, the number ofaccelerating cavities had to be increased as far as possible so as toprevent the beam from having to execute too great a number ofrevolutions within the cyclotron. The reason for this is that, when thebeam has to perform a high number of revolutions, this requiresincreased precision in producing the magnetic field and, in this case,it is preferred to use all the valleys to house the acceleratingcavities therein.

Consequently, the extraction devices in superconducting isochronalcyclotrons are in-hill rejected, which markedly complicates theextraction. A second drawback, due to the fact that high fields areobtained for superconducting cyclotrons, is that the extraction devicesconstituted by an electrostatic channel and/or an electromagneticchannel have seen their relative efficiency decreased and consequentlysecond-generation cyclotrons require extraction devices which are muchmore complex than those of first-generation cyclotrons.

In particular, the extraction devices of the known second-generationcyclotrons have the feature that they occupy almost an entire machinerevolution along which may be numbered two to three extractors followedby three to ten focusing elements.

In all compact isochronal cyclotrons having superconducting ornon-superconducting coils, in which the air gap between two hills isessentially constant, a decrease in the induction is observed which isfelt as from the first two thirds of the pole radius and which falls tohalf of its maximum value at the radial extremity of the hills (hillradius).

A first solution to prevent this decrease was proposed by choosing anappreciably greater pole radius than that at which the maximum energy isachieved, but as a result, the radial zone where the magnetic fieldcontinues to increase without being isochronal was also lengthened; thismagnetic field passes through a maximum and decreases beyond that. Theextension of this radial edge-field zone will also markedly complicatethe extraction.

OBJECTIVES OF THE INVENTION

The present invention aims to propose a novel configuration ofsuperconducting or non-superconducting compact isochronal cyclotron nothaving the drawbacks of the prior art.

A first objective of the present invention aims to provide asuperconducting or non-superconducting compact isochronal cyclotronwhich tends to prevent the attenuation of the vertical component of theinduction when the radial extremity of the poles is approached.

In particular, the present invention aims to provide an isochronalcyclotron where the non-utilizable field zone at the extremity of thepoles is reduced to a few millimeters.

A complementary objective of the present invention is to propose acyclotron which has a simplified extraction device, in particular in thecase of a superconducting cyclotron.

Other objectives and advantages will appear in the description whichfollows.

MAIN CHARACTERISTIC ELEMENTS OF THE PRESENT INVENTION

The present invention relates to a superconducting ornon-superconducting compact isochronal cyclotron in which the particlebeam is focused by sectors, comprising an electromagnet constituting themagnetic circuit which includes at least three pairs of sectors called"hills" where the air gap is reduced, these being separated by spaces inthe form of sectors called "valleys" where the air gap has a greaterdimension and which is energized by at least one pair of main circularcoils surrounding the poles of the electromagnet, this cyclotron beingcharacterized in that the air gap of the hills has an essentiallyelliptical changing profile which tends towards complete closure at theradial extremity of the hills (hill radius) on the mid-plane and which,more particularly, totally closes up on the mid-plane.

By the expression "tends towards complete closure" is meant theconfigurations where a small residual opening (preferably less than thevertical dimension of the accelerated beam) remains and theconfigurations where the closure of the elliptical profile of the airgap is complete in the mid-plane.

According to this latter configuration of the air gap of the hills(complete closure of the air gap), perfect continuity of the inductionover the entire radial extent of the hills is theoretically obtained inthe case where the magnetization of the iron is uniform (constantmodulus and constant direction), and this is so even in the case wherethe pole radius is equal to the hill radius.

In practice, with soft iron, this state of uniformity of themagnetization is achieved when the iron of the hills works atsaturation, that is to say when the induction in the iron of the hillsis greater than 2.2 tesla. In the case where the pole radius isapproximately (to within 1 mm) equal to the hill radius, perfectcontinuity of the induction in the air gap is then achieved overvirtually the entire extent of the air gap of the hills.

Nevertheless, there still remains a rise in the induction in thevicinity of the hill radius, because of the non-uniformity of themagnetization vector of the iron in the vicinity of this hill radius.

In order to prevent this phenomenon, provision is made to produce aclosure of the air gap in the mid-plane in the form of a magnetic shuntbetween each pair of hills. This shunt preferably has a radial thicknesslying between 2 and 10 mm so as to increase by this amount the poleradius with respect to the hill radius.

The closure of the air gap in the region of the shunt must not becomplete; in fact, it suffices for the residual air gap to remain smallcompared to the vertical dimension of the accelerated beams.

In addition to the fact that, according to this configuration, virtualperfect continuity of the internal induction is reestablished up to thehill radius, an extremely rapid decrease in the induction is alsoobserved outside, beyond the hill radius, which enables the system forextracting the particle beam to be greatly simplified.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will be better described with the aid of theappended figures in which:

FIG. 1 represents, diagrammatically, an exploded view of the mainelements constituting the lower half of a compact isochronal cyclotron;

FIG. 2 represents a sectional view of a cyclotron according to thepresent invention;

FIG. 3 represents a more detailed view of an air gap between two hillshaving the essential characteristics of the present invention;

FIGS. 4 to 11 are graphical representations of the value of the verticalcomponent of the induction as a function of the radius at the mid-planeof the air gap located between two hills for a cyclotron of the priorart (FIGS. 4 and 5) or according to a cyclotron of the present invention(FIGS. 6 to 11).

DESCRIPTION OF A PREFERRED EMBODIMENT OF A CYCLOTRON ACCORDING TO THEINVENTION

The cyclotron shown diagrammatically in FIG. 1 is a cyclotron intendedfor accelerating protons up to an energy of 230 MeV.

The magnetic structure 1 of the cyclotron is composed of a certainnumber of elements 2, 3, 4 and 5, made of a ferromagnetic material, andof coils 6 made of a preferably conducting or superconducting material.

The ferromagnetic structure consists:

of two base plates 2 and 2' called yokes;

of at least three upper sectors 3 called hills and of the same number oflower sectors 3' (see FIG. 2) which are located symmetrically, withrespect to a plane of symmetry 10 called the mid-plane, with the uppersectors 3 and which are separated by a small air gap 8; between twoconsecutive hills there exists a space where the air gap has a greaterdimension and which is called a "valley" 4; and

of at least one flux return 5 connecting, in a rigid way, the lower yoke2 to the upper yoke 2'.

The coils 6 have an essentially circular shape and are located in theannular space left between the sectors 3 or 3' and the flux returns 5.

These coils may be made of a superconducting material, but, in thiscase, it will be necessary to provide the necessary cryogenic devices.

The central conduit is intended to receive, at least in part, the source7 of particles to be accelerated, these being injected at the centre ofthe apparatus via means known per se.

FIG. 2 represents a sectional view of a cyclotron according to thepresent invention.

The essential characteristic of the cyclotron according to the presentinvention is constituted by the fact that the air gap 8 located betweentwo hills 3 and 3' has an essentially elliptical changing profile whichtends to close up on the mid-plane 10 at the radial extremity of thehills, called the hill radius R_(c).

Preferably, the closure is complete at the radius R_(c) or at the veryleast the residual air gap is less than the vertical dimension of thebeam.

According to a further preferred embodiment, shown in FIG. 3, a magneticshunt 9 has been placed, beyond the hill radius R_(c), between each pairof hills 3 and 3', which is in the form of a metal screen having aradial thickness lying between 2 and 10 mm and preferably of the orderof 6.5 mm.

In this case, the pole radius R_(p) and the hill radius R_(c) are nolonger coincident, the pole radius lying, of course, at the radialextremity of the magnetic shunt.

It is obvious that at least one magnetic shunt 9 is equipped with .atleast one opening 11 in order to enable the extracted beam to pass.Preferably, it is constructed obliquely with respect to the hill radius.

FIGS. 4 to 11 represent the vertical component B_(z) of the induction asa function of the radius γ in the case of a uniform magnetization M.

FIGS. 4 and 5 represent this variation in the case of a constant air gapb between two hills, as this is the case for a cyclotron according tothe prior art.

It is observed that, in this case, the vertical induction B_(z)decreases rapidly as a function of the radius γ and this is so alreadyfor a value markedly less than the pole radius R_(p).

This decrease is already felt as from the first two thirds of the poleradius and falls to half of its maximum value at the hill radius R_(c).

FIGS. 6 and 7 represent the variation in the magnetic induction B_(z) asa function of the radius γ in the case where the air gap has anelliptical shape closing up completely at the pole radius R_(c), in thetheoretical case of a uniform magnetization M.

In this theoretical case, perfect continuity of the induction isobserved for any radial distance less than the radius R_(c) and anextremely rapid decrease beyond R_(c), even in the case where R_(p)equals R_(c).

Nevertheless, as already mentioned previously, this case is theoretical;in reality, with soft iron, a non-uniformity of the magnetization M isobtained in the vicinity of the pole radius R_(p), which consequentlygenerates a rise in the induction, such as shown in FIGS. 8 and 9.

In order to avoid this undesirable effect, a magnetic shunt should beintroduced which obstructs the mid-plane and thus makes it possible toreestablish the uniformity of the magnetization and, consequently, thevirtually perfect continuity of the vertical induction for a radius lessthan the radius R_(c), as appears in FIGS. 10 and 11.

It should be noted that the value of the vertical component B_(z) (r) ofthe magnetostatic induction for a radius less than the radius R_(c)essentially depends on the value of the minor half-axis (b) of theellipse generating the profile of the air gap formed between two hills.

The main advantage of this configuration of the air gap for a cyclotronaccording to the present invention resides in the fact that the systemfor extracting the particle beamwill be greatly simplified compared tothe extraction system for cyclotrons according to the state of the priorart.

In particular, a cyclotron according to the present invention, which isintended to accelerate protons to an energy greater than 150 MeV, maypossess an extraction system composed solely of a single electrostaticdeflector followed by two or three focusing magnetostatic channels.

In the present case, these magnetostatic channels consist of soft-ironbars having a rectangular cross-section of small dimension andconsequently have a very low production cost.

In general, a cyclotron according to the present invention has theadvantage of a reduction in the volume of iron necessary for producingthe poles of the yoke compared to those of a cyclotron according to theprior art.

I claim:
 1. A compact isochronal cyclotron in which the particle beam isfocused by sectors, comprising an electromagnet having two poles, saidpoles defining a mid-plane and constituting a magnetic circuit whichincludes at least three pairs of sectors (3 and 3') called "hills" wherethe air gap is reduced, these being separated by spaces in the form ofsectors (4) called "valleys" where the air gap has a greater dimensionand which is energized by at least one pair of main circular coils (6)surrounding the poles of the electromagnet, this cyclotron beingcharacterized in that the air gap (8) located between the two hills (3and 3') has an essentially elliptical changing profile which tendstowards complete closure at the radial extremity of the hills, calledthe hill radius (R_(c)), on the mid-plane (10).
 2. Cyclotron accordingto claim 1, characterized in that the air gap (8) between two hills (3and 3') closes up completely at the hill radius (R_(c)) on the mid-plane(10).
 3. Cyclotron according to claim 1, characterized in that the airgap (8) between two hills (3 and 3') has a slight opening, at the hillradius (R_(c)), preferably less than the vertical dimension of the beamto be extracted.
 4. Cyclotron according to claim 2, characterized inthat a magnetic shunt (9) produced in continuity with the poles of theelectromagnet is placed between each pair of hills (3 and 3') beyond theradial extremity (R_(c)) of the hills.
 5. Cyclotron according to claim4, characterized in that at least one magnetic shunt (9) is equippedwith at least one opening (11) so as to enable the extracted beam topass.
 6. Cyclotron according to claim 4, characterized in that themagnetic shunts (9) are in the form of a metal screen having a thicknesslying between 2 and 10 mm and preferably of the order of 6.5 mm. 7.Cyclotron according to claim 1, characterized in that the extractionsystem associated with the cyclotron is composed of a singleelectrostatic deflector followed by preferably two or three focusingelectrostatic channels.
 8. A method for accelerating protons to anenergy greater than 150 MeV comprising subjecting the protons to amagnetic field provided by a magnetic circuit which includes at leastthree pairs of sectors (3 and 3') called "hills" where the air gap isreduced, these being separated by spaces in the form of sectors (4)called "valleys" where the air gap has a greater dimension and which isenergized by at least one pair of main circular coils (6) surroundingthe poles of the electromagnet, said poles defining a mid-plane, thiscyclotron being characterized in that the air gap (8) located betweentwo hills (3 and 3') has an essentially elliptical changing profilewhich tends towards complete closure at the radial extremity of thehills, called the hill radius (R_(c)), on the mid-plane (10). 9.Cyclotron according to claim 3, characterized in that a magnetic shunt(9) produced in continuity with the poles of the electromagnet is placedbetween each pair of hills (3 and 3') beyond the radial extremity(R_(c)) of the hills.
 10. Cyclotron according to claim 9, characterizedin that at least one magnetic shunt (9) is equipped with at least oneopening (11) so as to enable the extracted beam to pass.
 11. Cyclotronaccording to claim 5, characterized in that the magnetic shunts (9) arein the form of a metal screen having a thickness lying between 2 and 10mm and preferably of the order of 6.5 mm.
 12. Cyclotron according toclaim 10, characterized in that the magnetic shunts (9) are in the formof a metal screen having a thickness lying between 2 and 10 mm andpreferably of the order of 6.5 mm.
 13. Cyclotron according to claim 2,characterized in that the extraction system associated with thecyclotron is composed of a single electrostatic deflector followed bypreferably two or three focusing electrostatic channels.
 14. Cyclotronaccording to claim 3, characterized in that the extraction systemassociated with the cyclotron is composed of a single electrostaticdeflector followed by preferably two or three focusing electrostaticchannels.
 15. Cyclotron according to claim 4, characterized in that theextraction system associated with the cyclotron is composed of a singleelectrostatic deflector followed by preferably two or three focusingelectrostatic channels.
 16. Cyclotron according to claim 5,characterized in that the extraction system associated with thecyclotron is composed of a single electrostatic deflector followed bypreferably two or three focusing electrostatic channels.
 17. Cyclotronaccording to claim 6, characterized in that the extraction systemassociated with the cyclotron is composed of a single electrostaticdeflector followed by preferably two or three focusing electrostaticchannels.
 18. Cyclotron according to claim 9, characterized in that theextraction system associated with the cyclotron is composed of a singleelectrostatic deflector followed by preferably two or three focusingelectrostatic channels.
 19. Cyclotron according to claim 10,characterized in that the extraction system associated with thecyclotron is composed of a single electrostatic deflector followed bypreferably two or three focusing electrostatic channels.
 20. Cyclotronaccording to claim 11, characterized in that the extraction systemassociated with the cyclotron is composed of a single electrostaticdeflector followed by preferably two or three focusing electrostaticchannels.